1. Field of the Invention
The present invention relates to a spark plug used for providing ignition in an internal combustion engine.
2. Description of the Related Art
Conventionally, in order to enhance ignition performance of a spark plug, a method has been proposed in which a groove is formed on a surface of a center electrode or a surface of a ground electrode which faces a spark discharge gap. By employing a groove, a flame nucleus generated by ignition of an air-fuel mixture induced by spark discharge can grow greatly in volume at the groove portion before contact with the electrode, thereby alleviating a cooling action (flame-extinguishing action) which is exerted by the electrode. As a result, ignition performance is enhanced, thereby preventing misfire and impairment in combustion.
In recent years, calls for global environmental protection have been growing. Under these circumstances, stronger energy conservation, CO2 effluent control, and emission limitation on unburnt gas (hydrocarbon compounds) have been required. In order to meet such demands, automakers have been actively developing lean burn engines, direct-injection gasoline engines, low-emission engines, and the like. Furthermore, a lean burn engine has actively introduced therein an exhaust gas recirculation (EGR) system in which a portion of exhaust gas is recirculated into a combustion chamber so as to reduce negative workload to be done by the engine at the intake stroke, as well as to clean exhaust gas. In such applications, a spark plug must ignite an air-fuel mixture which is lean and contains a large amount of exhaust gas, which is an inert gas. Therefore, the above-mentioned conventional measures can no longer sufficiently cope with such applications.
Another technique for improving ignition performance of a spark plug is disclosed in Japanese Patent Application Laid-Open (kokai) No. S59-37684. Specifically, a distal end corner portion of a ground electrode faces a distal end corner portion of a center electrode in a positional relation so as to form a relatively large angle with respect to the axis of the center electrode; i.e., obliquely. This patent publication describes that sparking in such a direction as to intersect the axis of the center electrode improves ignition performance. Japanese Patent Application Laid-Open (kokai) No. S62-43090 or Japanese Utility Model Application Laid-Open (kokai) No. S58-74788 also discloses a spark plug in which a distal end of a ground electrode obliquely faces the corner of a distal end portion of a center electrode.
3. Problems Solved by the Invention
However, the spark plugs disclosed in the above-mentioned patent publications have the following problems.
(1) Since these spark plugs are configured such that the distal end of a ground electrode faces the corner of the distal end of a center electrode, a corner portion of the electrode is apt to be locally ablated. In order to further enhance ignition property, the spark plug disclosed in Japanese Patent Application Laid-Open (kokai) No. S59-37684 or Japanese Utility Model Application Laid-Open (kokai) No. S58-74788 is configured such that a distal end portion of a center electrode, together with a distal end portion of an insulator, protrudes from the end surface of a metallic shell. Such a spark plug exhibits a marked increase in electrode temperature, since the position of a spark discharge gap is located closer to a central portion of a combustion chamber, which assumes a higher temperature. Lean burn engines, direct-injection engines, and the like exhibit higher combustion temperature. Therefore, the above-mentioned electrode ablation at an edge portion is apt to proceed to a greater extent, thereby raising a problem that electrode life tends to expire earlier than in the case of an ordinary spark plug.
(2) In the spark plugs disclosed in the above-mentioned patent publications, a corner portion of the distal end of a center electrode and a corner portion of the distal end of a ground electrode are arranged such that their facing direction forms a relatively large angle with respect to the axis of the center electrode. Studies conducted by the present inventors have revealed that a spark plug of such configuration fails to yield the effect of enhancing ignition performance to such a marked degree as expected.